Diagnostics

Problem

What becomes confusing, fragile, or impossible without understanding diagnostics? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Node applied JavaScript’s event-driven model to servers and command-line programs, pairing a single process with nonblocking I/O.

Concept explanation

Diagnostics belongs to “Node is a host runtime”. diagnostics is inspected by introducing one realistic failure and proving root cause before repair.

For diagnostics, trace concrete input, state transition, output, and failure through a server-side runtime boundary involving process lifecycle, filesystem or network I/O, streams, and capacity.

One process owns an event loop, module graph, heap, handles, streams, signals, and explicit boundaries to the OS and network. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied server.mjs. Focus: Capture healthy diagnostics baseline from: Build CLI accepting argument, printing JSON, and setting meaningful exit code.

1. Run: node server.mjs > server.log 2>&1 & server_pid=$!; trap 'kill $server_pid' EXIT; sleep 1; curl -i http://127.0.0.1:3000/
2. Save baseline evidence. Use Node inspector, active handles, process signals, curl, CPU profiles, heap snapshots, and OS process tools.
3. Boundary case: Record diagnostics behavior at: Run with missing/Unicode/large input and inspect active handles.
4. Failure case: Diagnose this exact diagnostics failure before editing: Send SIGINT during work and perform bounded graceful cleanup.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: CLI separates stdout/stderr, reports exit status, and exits with no leaked handles. Starter-level baseline: Server prints its URL; curl receives 200, JSON content type, correct byte length, and a body containing topic and GET method.

Starter material

Create server.mjs, paste this exact content, then run the command below.

import { createServer } from "node:http";

const server = createServer((request, response) => {
  const body = JSON.stringify({ topic: "diagnostics", method: request.method });
  response.writeHead(200, { "content-type": "application/json", "content-length": Buffer.byteLength(body) });
  response.end(body);
});

server.listen(3000, "127.0.0.1", () => console.log("http://127.0.0.1:3000"));

STOP / CLEANUPSupplied command traps shell exit and stops server. If running server alone, press Ctrl+C.

Independent exercise

Prove root cause: diagnostics

1. Create second case from blank file: Record diagnostics behavior at: Run with missing/Unicode/large input and inspect active handles.
2. Then create controlled failure: Diagnose this exact diagnostics failure before editing: Send SIGINT during work and perform bounded graceful cleanup.
3. Use Node inspector · curl · SQLite CLI · process tools to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• Evidence identifies first divergence, repair changes only proven cause, and rerun restores: CLI separates stdout/stderr, reports exit status, and exits with no leaked handles.
• Server prints its URL; curl receives 200, JSON content type, correct byte length, and a body containing topic and GET method.
• Controlled diagnostics failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.